In U.S. Pat. No. 3,925,625 issued Dec. 9, 1975 to R. J. Angner et al, there is disclosed a key telephone line circuit which is connected in shunt across the tip and ring lead communication path between the central switching machine and the subscriber's station to control the HOLD, BUSY and IDLE conditions of the station and to provide the necessary visual and audible signaling to the stations. The line circuit disclosed in the above-identified patent operates to detect ac current flowing for ringing control, while also detecting the transient response of the system for hold and disconnect control. The transient signals are generated upon on-hook to off-hook or off-hook to on-hook circuit transitions.
Shunt control has several advantages, but the most important one is the perfect balance of the tip and ring circuitry at all times. Other advantages include the ease of application of music and tone on hold signals and the fact that the circuit can be removed for maintenance in any state but HOLD or RING without disrupting customer service. The shunt principle of HOLD/DISCONNECT detection is as follows: When a key station goes on-hook, two things happen sequentially:
(1) THE TIP AND RING IS DISCONNECTED AT THE TELEPHONE SET WHICH CAUSES A TRANSIENT SIGNAL ON THE COMMUNICATION PAIR, AND
(2) THE A-lead is broken.
When a line is placed on HOLD, the A-lead is broken before the tip and ring are disconnected at the telephone set. Thus, the HOLD/DISCONNECT distinction is made by detecting the presence of current flow (transient signal) before the A-lead is broken.
The shunt line circuit must also operate to detect ringing and to provide a hold impedance across the communication pair when the line circuit is in the HOLD mode. Implicit in the ability of the line circuit to detect ringing signals is the ability to distinguish such signals from noise, TOUCH-TONE signals, dial pulses, or other unknown signal disturbances which periodically appear on telephone lines.
Since, using the shunt principle, the line circuit impedance is always across the communication line in the IDLE, RING or BUSY modes there are a number of constraints that must be met in order for the line circuit to function properly. In the IDLE state the communication lead interface must not interfere with tests which are made by the Central Office (CO) or PBX. Such tests are routinely made to insure that at least one telephone ringer, and hence at least one telephone station, is connected to the line. Other tests are made to insure the proper insulation of the line. In the BUSY state care must be exercised to insure that the interface does not distort the signaling between the telephone station and the central switching machine and does not adversely affect the voice transmission over the communication pair.
Accordingly, it is an object of our invention to provide a key telephone line circuit having a communication path interface circuit capable of permanent connection across the communication path and capable of satisfying the precommunication period restraints as well as the communication period restraints.
It is a further object of our invention to provide a shunt line circuit without using inductive elements in the interface path while at the same time meeting the central switching machine precommunication period ringer test.
It is a further object of our invention to provide a key telephone line circuit capable of permanent connection to the communication lead pairs during the IDLE, BUSY, RING, and HOLD modes of the line circuit and capable of controlling ringing signal detection and hold abandon detection under divergent line parameters.